fix(goals): surface GitHub rate limit error in goal sync

[mallya-m]

What does this PR do?

Fixes silent failure when GitHub Search API rate limit is hit during goal sync. Now returns proper 429 with reset time, and the UI surfaces a user-friendly message.

Related issue

Closes #935

Changes made

• Detect 403/429 in commit search loop in /api/goals/sync
• Detect 403/429 in PR count fetch
• Parse X-RateLimit-Reset header and include reset time in error message
• Return { error, rateLimited: true } with status 429
• GoalTracker.tsx shows rate limit message instead of generic error

How to test

1. Trigger sync rapidly multiple times
2. When rate limited, UI shows "GitHub rate limit reached. Sync will resume at HH:MM"
3. Normal sync still works when not rate limited

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[mallya-m]

@Priyanshu-byte-coder could you please add gssoc:approved, level:intermediate, and type:bug labels? I already implemented identical rate limit handling in PR #602 so this follows the exact same pattern. Thank you!

[github-actions]

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